830-515-2002

Groundwork for a Concrete Slab

A concrete slab consists of a layer of concrete, usually reinforced with wire mesh, that rests on a drainage bed of gravel. Although the 4-inch reinforced slab on 4 inches of gravel described on the following pages satisfies many building codes, always check with local authorities for the correct specifications.

Building codes may also specify the degree of slope required for water runoff and whether a silt fence is needed during construction to limit soil erosion. Local zoning laws may dictate the location, design, and size of a slab.

GETTING STARTED

Selecting a site for a slab requires the same care as choosing one for a brick patio. When your plan is completed use the estimator on page 128 to calculate how much concrete and gravel you’ll use, based on the area of the slab and the thickness of the concrete plus the gravel bed.

To prepare the site for a rectangular slab, lay out boundaries with wood stakes and string as shown on page 130, and excavate the site. Dig 2 feet beyond the strings to accommodate form boards and braces, then proceed as shown here. Site preparation for a free-form slab appears on pages 135-137. Save sod and dirt to fill in at the sides of the finished slab.

EXPANSION JOINTS

Many local codes require a strip of asphalt-impregnated expansion-joint filler in the concrete every 8 to 10 feet of a slab’s length and between the slab and the house foundation (page 132). The purpose of the joints is to prevent damage as concrete expands and contracts with changes in temperature. Try to buy joint filler as wide as the thickness of the slab. If unavailable, somewhat narrower or wider filler is satisfactory.

FORMS TO SHAPE CONCRETE

Unless you plan on a decorative pattern of permanent form boards (page 134), build forms from inexpensive woods such as fir, spruce, or pine. Plywood, made flexible with the technique shown on page 136, is used to mold the curves of a free-form slab. Double-headed nails allow both types of temporary forms to be quickly disassembled.

SAFETY TIP

Wear goggles when nailing form boards together and joint filler to the house. Gloves protect your hands from blisters, splinters, and especially cuts when handling wire mesh.

TOOLS Common carpentry tools Spade Tamper Screed Rake 4-pound maul Line level Wire cutters MATERIALS 1-by-2s 2-by-4s Plywood (¾-inch) 2-by-2 stakes Mason’s cord Double-headed nails (2-inch) Common nails (3-inch) Spikes (6-inch) Masonry nails (1½-inch) Washed gravel (¾-inch) Expansion-joint filler Reinforcing mesh Binding wire Concrete blocks Powdered chalk Lath

CALCULATIONS FOR SLOPE AND VOLUME

Calculating slope

For a slope of ¼ inch per foot, use the formula above to determine the difference in height between the edge of the slab that abuts the house and the edge parallel to it. Multiply by 0.125 instead of 0.25 for a slope of ⅛ inch per foot.

Length of a side boundary line in feet

____________ × 0.25 = ____________ inches

Estimating cubic yardage

Use the estimator above to find the amount of concrete or gravel needed for a slab or drainage bed. The result in cubic yards—the bulk measure in which such materials are sold—includes an 8 percent allowance for waste and spillage.

Area in Thickness square in feet inches

_____ × _______ × 0.0033 = _______ cubic yards

TILTING A SLAB FOR DRAINAGE

TILTING A SLAB FOR DRAINAGE

Leveling the excavated area

Using a rake, break up any clods of earth; then, with a helper, pull an 8-foot-long 2-by-4 leveling board across the area to smooth it (above). Dampen the area with a hose, and compact the surface by pounding it with a tamper. You can rent one or build one from a 2-foot square of ¾-inch plywood with a braced handle, 4 feet high, made of 2-by-4s. After tamping, pull the leveling board across the area again, using it as a straightedge to make sure that the surface is reasonably even.

Establishing the grade

Mark the slab height—the sum of the slab and gravel-bed thicknesses—on the house wall, 2 inches outside a side boundary line. Center a 2-by-2 reference stake on the mark and drive it into the ground next to the house wall, bringing the top even with the slab-height mark. Center a nail on top of the stake and hammer it in partway. Measure along the side boundary line to a point 2 inches beyond the end boundary line, and drive a corner reference stake opposite the first. Loop a string around the second stake and tie it to the nail in the other. Hang a line level at the center of the string and adjust the string at the corner stake to make the string level. Use the formula on the preceding page to calculate slope, and mark the resulting distance on the corner stake, below the string. Lower the string to the mark, called a grade mark. Repeat for the other side of the slab. Connect the grade marks on the corner stakes with a third string to make an end grade line. Use that line and a reference stake near the house to establish a grade line within the slab for each expansion joint required.

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Adding support stakes

Mark the grade lines at 2-foot intervals. Directly below each marker, drive a 2-by-2 form-support stake deep enough so you can’t pull it out by hand and so the grade line just touches the top of the stake. Place stakes precisely to ensure that corners will be square and the slab sloped correctly. When all of the support stakes are in place, remove the grade lines and use a handsaw to trim the corner reference stakes to the height marked earlier for the grade lines. Then remove all the boundary-marker strings and stakes; leave only the support stakes and the corner reference stakes.

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Installing form boards

Cut a 2-by-4 form board 4 inches longer than the distance from the house to the outer edge of the reference stake. Use more than one if needed, butting the boards. Set a board against the inside faces of the support stakes, one end against the house. Rest the board on wood blocks so that the top edge is even with the tops of the stakes. Drive a 2-inch double-headed nail through each end stake into the board, using a maul as an anvil (left). Then nail the board to the intermediate stakes. Install a form board for the other side of the slab in the same way. Nail a form board for the end of the slab to its support stakes. Where this board abuts the side boards, toenail the joints with 3-inch common nails. For each expansion joint, cut a board ½ inch shorter than the distance between the house and the end board. Nail the board to its support stakes on the side of the slab that will be poured first, leaving a ½-inch space at the wall. Where boards abut (inset), drive a support stake 6 inches to each side of the joint. Nail the boards to the stakes, then nail a strip of ½-inch plywood over the joint.

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Bracing the form boards

Drive a support stake at the corners where end and side boards meet. Reinforce each support stake, including those flanking a joint, with a 1-by-2 brace nailed to a stake driven about 1 foot from the form board. Excavate the slab area along the end board to match the slope of the side form boards, then smooth and tamp.

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Screeding the gravel

Pour a layer of ¾-inch gravel into each section of the slab, allowing it to spill out under the form boards. With a helper, smooth the gravel by dragging a screed across its surface.

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TO MAKE A SCREED

TO MAKE A SCREED

The function of a screed is to assure a smooth gravel surface and a uniform slab thickness. To make the screed shown here, which is designed for a slab 4 inches thick, cut a 1-by-8 board 2 inches shorter than the distance between forms, and a 2-by-4 board 10 inches longer than this distance. Center the 1-by-8 on the 2-by-4. Offset the 1-by-8 board ¼ inch and nail the boards together. To the face of the 1-by-8, toenail two 2-by-2 handles, each 4 feet long and cut at a 30° angle at one end. Brace the handles with 2-by-2s nailed to the top of the 2-by-4.

Installing joint filler

Cut a strip of expansion-joint filler the length of the expansion-joint form. Every foot or so, hammer 3-inch nails through one side of the filler and bend the points slightly to anchor it in the slab. Set the filler against the form (above). Use 6-inch spikes, if necessary, to support the filler at the correct height, even with the top (inset). Nail another strip of filler to the house wall with 1½-inch masonry nails at 6-inch intervals, outlining any existing stairs. Butt the filler against the side form boards and work it into the ½-inch space between expansion-joint forms and the house.

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Laying wire mesh

Wearing gloves, unroll mesh over the gravel. Begin at the outer edge of the slab, and leave 2 inches between the mesh and the form boards. Weigh down the mesh with concrete blocks as you go. When you reach the other side of a section, use wirecutters to trim the mesh 2 inches short of the form board. Then turn the mesh over, and walk on it to flatten it. Cover the gravel in the section you intend to pour first, allowing panels of mesh to overlap 6 inches. Cut the mesh to fit around steps or other obstacles, then tie panels together with binding wire.

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THE APPEAL OF PERMANENT FORM BOARDS

THE APPEAL OF PERMANENT FORM BOARDS

In most instances, form boards become scrap lumber after a slab is poured. A decorative option, however, is to leave them in place as a frame around the slab or, as shown here, to lay out an attractive pattern of interior form boards. For this purpose, pressure-treated or other weather-resistant wood is superior to the pine or spruce used for temporary forms.

Installing permanent form boards follows the same principles used for setting up temporary boards—with a few differences. When laying out the perimeter for such a slab, take into account that the form boards will contribute to its length and width. Set up and brace perimeter form boards as shown on pages 130-131, but drive support stakes for interior boards as well as perimeter boards an inch below the planned surface of the slab so that the concrete will conceal them.

Establish the slope of the slab as shown on page 129. Each interior board that will lie perpendicular to the house needs a grade line. Boards oriented parallel to the house each require a string set up in the same way as an end line (page 129).

Permanent form boards replace expansion joints within the slab but not the one along the house. Boards that cross from one side of the slab to the other are called primary form boards. Next to the house, use stakes to secure the ends of primary form boards; nail through perimeter form boards to secure other ends.

Secondary form boards are shorter than the width or length of the patio. Where secondary form boards meet perimeter or primary form boards, they can be face-nailed. Where two secondary boards meet, nail one to the other and toenail both to a 2-by-4 stake driven into the ground under the joint.

Before pouring the slab, drive 3-inch nails halfway into all the form boards to help anchor them in the concrete, and cover their top edges with heavy-duty tape to prevent concrete stains.

CURVED FORMS FOR A ROUNDED SLAB

CURVED FORMS FOR A ROUNDED SLAB

Excavating the slab area

Establish the slab shape with a garden hose, then mark the shape on the ground with powdered chalk. Remove the hose, and excavate the area a distance 2 feet beyond the chalk line. Dig to a depth equal to the thickness of the drainage bed plus the thickness of the slab. Dig back and forth across the area in parallel rows, then smooth and tamp the soil in the bottom of the excavation. Establish drainage away from the house, first by setting up boundary lines and grade lines for a rectangle a foot or two larger in each dimension than the slab’s maximum length and width (inset). Tie intermediate grade lines at 3-foot intervals between the side lines, and between the wall of the house and the end line; at the house, tie the strings to masonry nails. Drive 2-by-2 support stakes into the ground around the slab perimeter, 2 feet in from the edge of the excavation. Space the stakes 2 feet apart along gradual curves, 1 foot apart around sharp curves. Add support stakes for any interior forms required, spacing them at 2-foot intervals. Set the tops of the stakes even with the strings.

TAKING THE STIFFNESS OUT OF PLYWOOD

TAKING THE STIFFNESS OUT OF PLYWOOD

The rigidity of plywood—layers of wood glued together under pressure—results from orienting the grain in each layer perpendicular to the grain in its neighbors. Yet you can make even ¾-inch plywood bend enough to serve as forms for a curved slab. With a circular saw, cut strips about 4 inches wide with the outer grain running lengthwise. Then set the saw for a depth of ½ inch and saw across the strip. Check that the blade cuts through no more than three of the plywood’s five layers. Adjust the saw as necessary, then cut grooves across the strip every inch or so.

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Installing form boards

To measure for curved form boards, first cut a strip of lath 4 to 8 feet long. Tack it to support stakes, even with the tops and butted against the house. Mark the lath at the midpoint of the support stake nearest the end, then pry off the lath and transfer the mark to a strip of ¾-inch plywood cut as wide as the slab depth. Trim the plywood at the mark. Cut slots in the plywood to make it pliable (above), then nail it to the support stakes. Proceed around the slab perimeter in this fashion, butting sections of plywood at support stakes (inset). Install expansion-joint forms as necessary (page 132).

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Sloping the forms

Work around the curved form, lifting or hammering down the boards and stakes until the top edge of the form just grazes the grading lines. Repeat for the straight forms of the expansion joints. When both the curved and the straight forms are adjusted, secure the support stakes with braces (page 131). Correct the depth of the excavation to match the slope of the grade, then spread a gravel drainage bed 4 inches thick. Install joint filler as shown on page 132.

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Cutting mesh to fit the form

Unroll a length of wire mesh over a section of the form, letting it overlap the curve of the form boards where necessary. Anchor the mesh temporarily with cinder blocks, then cut the mesh along the curve, 2 inches inside the form. Flatten the mesh either by bending or by removing it from the form and walking it flat. Cut additional sections of mesh, allowing each to overlap the previous section by 6 inches. Flatten the sections and tie them together with binding wire.

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